drm/gem: Fix mmap fake offset handling for drm_gem_object_funcs.mmap

Commit c40069c ("drm: add mmap() to drm_gem_object_funcs")
introduced a GEM object mmap() hook which is expected to subtract the
fake offset from vm_pgoff. However, for mmap() on dmabufs, there is not
a fake offset.

To fix this, let's always call mmap() object callback with an offset of 0,
and leave it up to drm_gem_mmap_obj() to remove the fake offset.

TTM still needs the fake offset, so we have to add it back until that's
fixed.

Fixes: c40069c ("drm: add mmap() to drm_gem_object_funcs")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024191859.31700-1-robh@kernel.org

drivers/gpu/drm/drm_gem.c

@@ -1106,6 +1106,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
 		return -EINVAL;
 
 	if (obj->funcs && obj->funcs->mmap) {
+		/* Remove the fake offset */
+		vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
+
 		ret = obj->funcs->mmap(obj, vma);
 		if (ret)
 			return ret;

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -541,9 +541,6 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
 	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
 	vma->vm_ops = &drm_gem_shmem_vm_ops;
 
-	/* Remove the fake offset */
-	vma->vm_pgoff -= drm_vma_node_start(&shmem->base.vma_node);
-
 	return 0;
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_mmap);

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -480,6 +480,13 @@ EXPORT_SYMBOL(ttm_bo_mmap);
 int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
 {
 	ttm_bo_get(bo);
+
+	/*
+	 * FIXME: &drm_gem_object_funcs.mmap is called with the fake offset
+	 * removed. Add it back here until the rest of TTM works without it.
+	 */
+	vma->vm_pgoff += drm_vma_node_start(&bo->base.vma_node);
+
 	ttm_bo_mmap_vma_setup(bo, vma);
 	return 0;
 }

include/drm/drm_gem.h

@@ -159,8 +159,9 @@ struct drm_gem_object_funcs {
 	 *
 	 * The callback is used by by both drm_gem_mmap_obj() and
 	 * drm_gem_prime_mmap().  When @mmap is present @vm_ops is not
-	 * used, the @mmap callback must set vma->vm_ops instead.
-	 *
+	 * used, the @mmap callback must set vma->vm_ops instead. The @mmap
+	 * callback is always called with a 0 offset. The caller will remove
+	 * the fake offset as necessary.
 	 */
 	int (*mmap)(struct drm_gem_object *obj, struct vm_area_struct *vma);